1. Field of the Invention
This invention relates to a method of and an apparatus for stably forming a proper image.
2. Description of the Prior Art
For example, an exposure light source, a corona charger and a developing unit are generally used to form an image on a photosensitive medium. Corona charging is effected with discharging taking place in the air and so, the corona discharging condition is varied by variations in environmental conditions such as humidity, temperature and atmospheric pressure and by contamination of the discharging wire attributable to the suspensions in the air, and accordingly the quantity of electric current directed toward a member to be charged is varied to thereby vary the potential of the member to be charged. This results in pronounced variations in the developed image in the electrophotography wherein an electrostatic latent image is formed on the member to be charged and developed into a visible image. To stabilize the condition of the formed image, the charging effected on the member to be charged by corona discharge must be made stably or some method must be adopted to compensate for the variation in the latent image potential resulting from the variation in charging on the member to be charged.
A method disclosed in U.S. Pat. No. 2,956,487 is known as the method for providing a stable image. It is also known to use a constant current source as the power source for the corona charger and impart a constant quantity of corona discharge current to the charged member.
However, where the surface potential before the charged member is subjected to said charging is not constant or where the electrostatic capacity between the charged member and the earth is not constant, the charged member cannot be charged to a constant potential.
The reason will hereinafter be described. Generally, if the electrostatic capacity of the charged member is C, the variation in surface potential of the charged member resulting from charging is .DELTA.V and the charge imparted to the charged member by charging is .DELTA.Q, then .DELTA.Q may be expressed as .DELTA.Q=C.DELTA.V. In the charging method wherein a constant quantity of effective corona discharge current is imparted to a charged member, .DELTA.Q is a constant value determined by the charging time and the effective corona discharge current.
Here, if the electrostatic capacity C of the charged member is varied with time or the like, the amount of variation .DELTA.V of the surface potential of the charged member resulting from the charging is varied since .DELTA.Q is constant and so, the surface potential of the charged member after subjected to the charging is not constant. Also, even if the amount of variation .DELTA.V of the surface potential resulting from the charging is constant, the surface potential of the charged member after being subjected to the charging is not constant unless the surface potential of the charged member before being subjected to the charging is constant.